


The Matoufly Effect

by niteynyx



Series: Nitey's Commissions [44]
Category: Fate/Zero, Fate/stay night & Related Fandoms, Fate/stay night (Visual Novel), Fate/stay night - All Media Types
Genre: Alternate Universe, Anal Sex, Cuckolding, Extremely Dubious Consent, F/M, Plot What Plot/Porn Without Plot, Porn With Plot, Rape, Rape/Non-con Elements, Rough Sex, Vaginal Sex, tbh i have no idea what to label 'fucking your friend's mom in an alternate universe'
Language: English
Status: In-Progress
Published: 2021-03-15
Updated: 2021-03-15
Packaged: 2021-03-23 18:02:34
Rating: Explicit
Warnings: Rape/Non-Con
Chapters: 1
Words: 4,116
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/30059421
Author URL: https://archiveofourown.org/users/niteynyx/pseuds/niteynyx
Summary: The flapping of a butterfly’s wings can supposedly cause a typhoon to take place on another continent. Zelretch casually and drunkenly fucking with reality can have a similiar effect -- like preventing the tragedy of the Fourth Holy Grail War, stopping the Fifth from every happening, and letting Matou Shinji finally get a chance to fuck his former friend’s adoptive mother, Emiya Irisviel. Anonymous commission.
Relationships: Matou Sakura/Matou Shinji, Matou Shinji/Irisviel von Einzbern
Series: Nitey's Commissions [44]
Series URL: https://archiveofourown.org/series/1896736
Kudos: 8





	The Matoufly Effect

The butterfly effect: the idea that even the smallest things can have non-linear impacts on far more complex systems, like a butterfly flapping its wings and causing a typhoon. The official term for the theory was coined by a meteorologist named Edward Lorenz in the nineteen sixties, who noticed slight changes in his computer weather models -- the literal equivalent of a butterfly flapping its wings -- could drastically affect the outcome, without any way of predicting the scale or extent of the chaos.

The Clock Tower was deeply troubled by Lorenz’ paper. Though they knew the butterfly effect by another name, they disliked that the scientific population had become aware of the Base Law of True Magic. Modern members of the Mage’s Association distilled it down to something far more plain than Lorenz. Shit happens and you’ll probably never figure out why it happens, even if you’re able to time travel, manipulate events and observe how each slight change shifts events drastically.

Of course, for  _ them _ time travel and omnipotence was just as much a theory as the butterfly effect itself. Zelretch was careful to keep things that way. His Second Magic went far deeper than just letting him travel between parallel worlds, giving him godlike abilities over time and space that perhaps made him the single most dangerous creature in existence throughout the cosmos. If anyone learned the true extent of his enemies -- well, he’d have a lot more enemies trying to kill him or steal his secrets, not to mention people begging him to make changes to reality’s delicate balance for their sake.

He took care not to abuse his great power, taking the inherent responsibility they prevented very seriously. There were numerous opportunities for Zelretch to prevent great tragedies and wars, but without being able to predict how they would change the world he always stayed his hand; he alone shouldered the burden of knowledge that such things did not have to be, out of fear of the unknown.

When the Mage’s Association voted to assassinate Lorenz and use powerful magic to erase knowledge of the butterfly effect theory from the public zeitgeist, he used his veto, simply explaining that no one would take it seriously, someone else would inevitably discover it. And, of course, scientists were even less likely than mages to ever master the Base Law of True Magic. Those  _ were _ his true feelings on the matter, but not quite his motive in exercising his veto for the first time in a decade. It was to Zelretch’s advantage if the butterfly effect faded into science fiction. The idea would linger for a few years, then become nothing more than a fantasy due to the impracticality of studying it.

It played out exactly how he expected it to, of course. Most things did. He was pleased beyond measure when some forty years later, Eric Bress and J. Mackye Gruber headed a movie that would end any chance of anyone seriously looking into the Base Law --  _ the Butterfly Effect _ . He didn’t watch it the year it came out -- he was  _ far _ too busy exploring an alternative reality where cat girls had destroyed human society and installed a new matriarchal order. It was a wild time. 

He  _ did  _ watch it the next year, sequestered away in a hotel room and sauced up on sake, one of the only movie rentals available to him. He hated every moment of it, barely able to understand the plot in his intoxicated state -- something he unfairly blamed on the film’s leading man, Ashton Kutcher. Were Zelretch sober, he might have enjoyed the movie, or he might have at least just turned it off and moved on with his life, knowing he couldn’t do anything about it. But Zelretch wasn’t sober, and he  _ could _ do something about it. He employed his Second Magic, his Kaleidoscope, traveling through time in a drunken rage until he found the pivotal moment that turned Kutcher into an actor. He set the man on a different path entirely in 1992, by simply showing up and punching him in the face right before he auditioned for his first modeling gig. The company just wasn’t interested in a child star with a black eye.

That path would eventually lead Ashton Kutcher becoming the President of the United States, and it led to another man taking the leading role of  _ the Butterfly Effect _ : Steve Carrell. A few years later, Carrell would later head the dark and gritty reboot of a certain popular UK TV show as Michael Scott, district manager and mob boss. Zelretch wasn’t satisfied until he sat down and watched the movie through.

Finding it to be a much better experience, and vastly preferring this less comedic Carrell, Zelretch polished off his bottle of sake and blacked out drunk on his hotel room’s floor. When he woke up, he didn’t remember anything that happened the previous night -- and considering all of his world hopping, he barely noticed the monumental changes his meddling had on reality and history.

For a lot of people, the world was better for it. Take the five hundred citizens of Fuyuki City, for instance, who  _ would _ have died in a fiery cataclysm at the conclusion of the Fourth Holy Grail War, when Emiya Kiritsugu destroyed the Holy Grail. In this changed world, this alternate timeline created by Zelretch’s drunken fuckery, Kiritsugu found a way to cleanse the Grail of Angra Mainyu’s influence and use it to grant his true wish: a life with Irisviel, who wasn’t born at all but rather created as a homunculus, with no purpose other than to serve as the vessel for the Greater Grail.

Irisviel lived. Kiritsugu lived. Five hundred people and a great swathe of Fuyuki survived its dark fate. Sure, Shirou’s parents tragically passed away due to carbon monoxide poisoning a year later, but that was  _ almost _ okay because Kiritsugu still saved him, still (somewhat questionable) stuck Avalon in him, and still adopted him. The Einzberns even cut their losses and let Kiritsugu and Irisviel live in spite of their betrayal. It was really all for the best. 

Perhaps Shirou would have mourned the future that might-have-been if he knew that he would have lost his virginity to the hottest girl in his school  _ and _ gone on to fuck his best friend Sakura, the itty bitty titty female incarnation of King Arthur  _ and _ a buxom gorgon rocking librarian vibes, but he never knew any better. He never even learned about the Fourth Holy Grail War, because the Fifth wouldn’t happen for another fifty years.

Some things didn’t change for the better, of course. Sakura was still adopted into the Matou family, and Shinji was still a complete and utter prick, but you take the good with the bad.

*****-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-*****

“Sakura,” Shinji sing-songed as he prowled the halls of the Matou family’s mansion. “Your step-brother is coming to pla-ay,” he hummed out as he swaggered and swayed his way down the halls. Any other day, he wouldn’t be so loud or so blithe, but Zouken was out of the house on unspecified magus business for the summer that Shinji knew better than to inquire about. That meant the two of them had the house to themselves, and for Shinji that meant he could do  _ whatever _ he pleased to his dear sister. Not that Zouken being home would stop him from that. The old man really didn’t care what either Shinji or Sakura got up to, so long as they were obedient to his commands. The difference was that he could do whatever  _ wherever _ in the house, however loud he cared to be.

And he had every intention of making his step-sister howl to celebrate the first day of their first summer as high school graduates. The blue-haired bastard couldn’t wipe the perverse grin from his face, swinging her new leash in one hand, a chain training collar fit for a dog attached to one end. He plotted everything out down to the tiniest detail. He knew exactly how he was going to pin her down when he found her. The box cutter tucked in his left pocket would make quick work of her clothes; only when she was naked would he get the collar on her, making her crawl all throughout the house and throughout the gardens. He would wear her out, leave her arms and legs trembling, then begin dragging her along when she couldn’t keep up any longer.

And when he was bored of that, he was going to drag her into Zouken’s room and break in her ass on the old man’s bed. He’d make her clean everything up and destroy the evidence, of course -- with her tongue first, and then actually,  _ really _ deep clean the place. It would be a small act of revenge against their controlling grandfather, a step up from what Shinji usually did when he just  _ needed _ to act out. Things like going into another room, crawling under a blanket and flipping the bird in Zouken’s general direction while crying.

“Saa-kurr-uh,” Shinji called, finishing his circuit of the house where he began his search, at the door of Sakura’s bedroom. Nothing, and she knew better than to hide from him. Last year, when he really began using her as his personal cumdump, he spent a month training her to come when he called her name. She had it down pat on day one, of course, but being the sadistic little asshole that he was, Shinji cruelly dragged it out, coming up with lewder and crueler punishments to force on her with increasingly arbitrary rationale.

He checked inside her room one more time, then grunted his dissatisfaction and dropped the leash and crossed his arms. If he was Sakura, and he didn’t come straight home after his morning chores to get fucked, where would he go? The protagonist (of his own story, certainly not anyone else’s) thought long, and he thought hard, and when he came to his conclusion he felt dense as a brick.

The only place Sakura could  _ possibly _ be was the Emiya household.

*****-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-*****

“Yeah,” Shirou answered Shinji flatly, honest to a fault, even as he stood in the Emiya household’s doorway and prevented Shinji from entering. “She’s here… but I don’t think you should be.” Right up until last year, Shinji and Shirou were as close as brothers -- and then Shirou saw Shinji slap Sakura for no other reason than not smiling at him. That moment redefined their relationship and recontextualized everything Shirou knew about Shinji. Naive and trusting, Shirou took the explanations for Sakura’s bruises at face value until he saw Shinji make one.

Not that Sakura ever admitted to it. She knew better than that. Shirou still had no idea how much of a whore the seemingly demure and innocent girl was -- how lewd her body was with its fat tits and ever-wet pussy. Still, she would occasionally go to the Emiya household to get away from him, knowing that Shirou wouldn’t let him in.

Before Shinji could verbally destroy Shirou for his  _ gall _ , a woman’s throaty voice called out from behind him. “Who is it, honey?” Emiya Irisviel showed her face a moment later, smiling like the pale-haired angel she was. Shinji felt his heart skip a beat. Unsettlingly red eyes be damned, Shinji had always harboured a deep crush on Shirou’s adoptive mother. She was the very picture of beauty to him. Some people might have been satisfied with a wet and willing woman like Sakura, with her big breasts, ample ass and womanly hips. But Irisviel was the one that fantasized about, slim and graceful, somehow all the more sexier than Sakura with her slightly more modest figure. “Oh, Shinji-kun!”

Irisviel always loved Shinji, just not in the way that Shinji wanted her to. She didn’t see the scumbag Shirou did; she only saw him as a polite young man, her son’s best friend from another line of magi, and perhaps her future son-in-law if Sakura and Shirou finally tied the knot. Shirou never had the heart to reveal what he knew to her -- or his father, for that matter, who he knew may very well have resorted to vigilante justice. Her beautiful, angelic smile brightened. “Are you here for Sakura-chan? We’re about to have dinner. You’re more than welcome to join us.”

“Actually, Shinji-san was--” Shirou cut in, but he was never as quick with his tongue as Shirou.

“I’d love to,” Shinji interjected, for no other reason than to piss Shirou off and spend some time with the woman he pined for. He grinned, already thinking about how he’d take out his sexual frustrations on Sakura and punish her. For the rest of the summer, he’d make sure she didn’t go anywhere without his express permission and knowledge.

Shirou turned back towards Shinji and glared a warning at him, unseen by his mother. Shinji only smiled and brushed past his old friend, going out of his way to bump him with his elbow. “You can borrow Kiritsugu’s slippers. He won’t be back home for a few days,” Irisviel said, already heading back inside towards the kitchen, “We’ll be ready in a minute!”

Shinji stopped in his tracks and mouthed those words.  _ He won’t be back home for a few days _ . … But he stopped himself from dwelling on those dangerous words, swallowing and clearing his throat. Shirou watched him like a hawk while he took off his shoes and tucked his feet into the slippers belonging to the man of the house -- the guy whose intensity scared the shit out of Shinji. The guy who got to come home and fuck Irisviel every night. He shifted the fit of his pants awkwardly as he headed inside, well aware he was already popping a telling stiffy.

*****-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-***-*****

The dinner was, as always, delicious. Irisviel’s cooking rivaled her beauty… not that Shinji would ever want to stick his cock in it. Throughout the meal, Shinji tried to push his libido to the side and just focus on what he would do to Sakura later that night, but then Irisviel  _ had _ to open her gorgeous lips, her perfect dick pillows, and ask Shirou if he could take Ilya to her ballet practice after walking Sakura home. She made it abundantly clear that he would be stuck there for three hours. And Shirou, bless his earnest heart, simply agreed, unaware of what was on Shinji’s mind.

He decided to go for it.

When dinner was over and the dishes were put away, Shinji headed out ahead of his former friend and the two girls, walking around the neighborhood until he caught a glimpse of them leaving. He took a deep breath, then headed back to the house. He showed himself inside, taking off his shoes and helping himself again to the slippers worn by Irisviel’s husband. The vacuum cleaner thrummed loudly, making it easy to find her in the living room, taking advantage of her alone time to tidy up. Her back was turned to him -- and it would have been  _ so _ easy for him to creep up behind her, but he didn’t want that for their first time.

“Irisviel-san,” he called out. 

The slim and motherly angel jolted with surprise, blinking as she looked over her shoulder at him. Her pure smile soon resurfaced, and she killed the vacuum’s power. “Shinji-kun. What are you doing back here?”

“I left something here,” the blue-haired bastard told her, smiling crookedly as he stepped towards her. Though his expression was calm and arrogant, his heart was thumping away in his chest with nervousness. After  _ years _ of lusting after Irisviel, he had finally had his chance to make her his. Its rapid beats only seemed to thunder harder as he got closer to her. 

She was wearing a simple sundress that complemented her pale skin and hair, all black with yellow floral patterns; her hair was pulled back in a thick, long ponytail for her housework. He glanced down at her dress, and couldn’t help but make the comment he had been holding back all night. “You’re a busy little bee, aren’t you?”

“You could say that,” Irisviel laughed kindly. “What did you forget? I’ll help you find it,” she assured him, though her warm and sunny smile suddenly froze. Her eyes widened slowly, and she glanced down at where Shinji’s hand had gone, barely reacting as he squeezed her breast through the sundress’ fabric.

“You know,” Shinji drawled, his racing heart calming as hers began to pick up. “Some honey. I think you’ve got it in your pot for me,” he told her with a cruel crook to his lips. He slid his fingers down, finding her tight and hard nipple and pinching it through her sundress. “The day’s over, so you took off your bra and let your worker bees fly free, huh?” he jeered.

The real reason that Shinji was here had become abundantly clear. Irisviel’s cheeks quickly grew pink, her wide red eyes lifting from his fingers to meet his gaze, embarrassed by his words and frightened by the sadism in his eyes. “Shinji-kun,” she whispered, “you need to go right now. I’m--”

He stepped into her, leaning down and kissing the lips he had fantasized about for so long just the way he dreamed about to, laying claim to her unwilling mouth savagely. She shut up, only able to gasp her shock into his mouth. There  _ was _ a time when Shinji was a purer boy, when he might have wanted Irisviel to fall in love with him, but that time was long past, and that purer boy was long dead thanks to Zouken’s influence. Irisviel resisted him every step of the way, only making Shinji’s cock harder -- something she could feel pushing against her stomach now. She grabbed at him with one hand, but his fingers caught her wrist. 

The kiss didn’t last long. He wasn’t interested in it, only sullying another man’s sacred ground. Shinji pulled away from her and grinned, half-pulling and half-dragging her over to the table, where less than an hour ago she had invited him to eat with her family. “Shinji-kun,” she whimpered, beginning to beg. “Stop this. You’re not--”

“Oh,” Shinji laughed, “I  _ am _ .” Not that he gave any thought to where she was going with that. He grabbed at her and kicked at her ankles, her yelp music to his ears. He forced her down to the low table, pressing her body flat against it, pressing her captured arm to the small of her back in an effective arm lock. There were things on the table her free hand  _ could _ have grabbed, things she could have used to swing back and attack him. “Hold still, now. I don’t want to ruin that beautiful skin you’ve got.” 

Though Irisviel had survived the Fourth Holy Grail War, she wasn’t a feisty woman -- and she didn’t have it in her to hurt the young man she watched grow into this monster. The young man she still thought was her son’s best friend. She closed her eyes and submitted rather than continue fighting, her breath coming in rapid little gasps, barely keeping herself from outright panic. She half-heard and half-felt his box cutter slicing her sundress wide open, ruining the garment. Her core tightened reflexively when she felt him at her panties, already slightly damp from her body’s natural reaction to the situation and stimulus. He cut those off, too, leaving her bare and vulnerable to him. 

“What-- oh, fuck,” Irisviel gasped and then groaned, opening her eyes and glancing behind herself to see what he was doing to her pussy, half-expecting her traitorous pussy to be filled with his cock already. Instead, he was shoving his fingers deep inside her, pushing her ruined panties in as far as he could. Her face burned brighter and she closed her eyes, not wanting to give him the satisfaction of knowing how humiliated she was, of knowing how much her pussy’s wetness was shaming her.

“Tomorrow night,” Shinji whispered to her as he drew his slick fingers out of her, “I’m going to come back and fuck your little cunt raw, Irisviel-san. And when I’m done, I’m going to fill your womb up with a little baby brother for Shirou-kun and Ilya-chan,” he taunted -- spur of the moment, just something to nettle her with. He didn’t want any kids.

Irisviel bit down on her bottom lip hard, though she felt a hint of hope raising in her belly. If he wasn’t going to fuck her tonight, that meant she would have a chance to call the cops -- or, no,  _ Kiritsugu _ . He would come back and deal with this right away for her. He always protected her. As the situation devolved, her frantic mind thought less and less about the consequences of harming her son’s best friend, of harming Zouken’s only blood heir.

“Tonight,” Shinji hummed gleefully, “I’m just going to give you a reminder to tide you over until I come back.” Irisviel’s eyes opened to tiny slits, and she glanced again over her shoulder, confused until she saw exactly what hole Shinji was lining his meaty cock up for.

“Don’t--” Irisviel tried to squeak out, aware that some people viewed the ass as a fuckable hole but never having any interest in it herself. That single word might as well have been giving Shinji permission to go ahead, for all the good it did her. He drove himself into her ass, claiming it roughly and mercilessly in a way that might break an ordinary woman. But she was born a homunculus. Her body could take it. The Einzberns were made to take it.

Her mind couldn’t. Irisviel gave Shinji the scream he had been longing to hear, high and desperate, her free hand clawing wildly at the table as though she hoped to pull and claw herself away from it. The fingers of her captured arm’s hand curled inward, scratching savagely at her palm, one of her long fingernails breaking rather than bending. Before it could hit its crescendo, before it could truly peak and perhaps alarm their neighbors, Shinji’s other hand reached forward and grabbed her head, muffling her by shoving her face against the table, holding it down against it.

He fucked her, hard and brutal, his hips slapping against hers, his balls continually clapping against her pussy and sending shocks of pleasure that confused all the pain and discomfort she was feeling. The blue-haired bastard kept going until Irisviel finally went limp under him, her mind trying to escape the situation her body faced while it still could. It happened faster than he would have liked, which was a disappointment -- but then again, he had to cum far faster than he would have liked as well, giddy with his success.

Shinji pulled his cock out of Shirou’s mother, spraying his load all over her ass and her back with soft grunts and growls, utterly satisfied. Where other men might have regretted their actions in their post-nut clarity, Shinji only felt smug as he stood up and observed his handiwork, his seed painted all over her sleek and sculpted back. Now came one of the most important parts of any such encounter -- the aftercare.

But like every nice thing Shinji did or every nice thought Shinji had, his idea of aftercare was nothing more than a perverse parody. He grabbed the vacuum cleaner and switched it back on, putting its end to the cum-splattered curve of Irisviel’s ass and starting to run it down her back. He took care to make sure he sucked it all up with the appliance, laughing to himself as he watched her twitch and shudder at the feeling. “Let me do you a favor, Irisviel-san,” Shinji hummed as he finally finished, crouching down. It took him a second to do, but he managed to prop the vacuum cleaner up in such a way that the attachment on its head pressed up against her pussy, the sucking sensation making her hips twitch and flinch.

Maybe she would cum from it. Maybe it would suck her panties out. Shinji didn’t really care. He stood up and tucked his softened cock away, zipping up and taking a second to make himself presentable. As a final touch, he kicked off Kiritsugu’s slippers and dropped them on Irisviel’s shuddering back. “Don’t forget to finish cleaning before the kids get home, Irisviel-san,” he told her chipperly. “And don’t forget that I’ll be back tomorrow.”

He left without looking back, confident she would do as she was told.

Humiliated, she did, too scared of what Shinji -- or Zouken -- might do if she didn’t.

**Author's Note:**

> Thanks for reading! Want to get updates, support my work or contact me? You can join my Discord server here to get previews of my work: https://discord.gg/2kpsyxb
> 
> Twitter: https://twitter.com/niteynyx  
> Email: niteynyx@gmail.com  
> Discord: niteynyx#8654


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